THE TWO COMETS OF THE YEAH 1868. 151 



appears as a light roundish cloud with a point of 

 brighter light near the centre. As it approaches the 

 sun the comet appears to grow considerably brighter 

 on the side turned towards him. An emanation of 

 light seems to proceed towards the sun for a short 

 distance and then to curl backwards and stream out 

 in a contrary direction. Gradually the backward 

 streaming rays extend to a greater distance the 

 nucleus continuing to throw out matter towards the 

 sun. Thus the tail is formed ; and it is often thrown 

 out to a distance of many millions of miles in a few 

 hours. 



One of the most singular facts connected with the 

 approach and recession of a comet, is the peculiarity 

 that the comet grows gradually smaller and smaller as 

 it approaches perihelion, and swells out in a corre 

 sponding manner as it passes away from the sun. The 

 comet of 1652 was observed by Hevelius to increase so 

 rapidly in dimensions as it passed away from the sun, 

 that between December 20 and January 12 its volume 

 had increased in the proportion of about 13,800 to 1. 

 When it was last visible this comet exceeded the sun 

 in volume. This observation, on which much doubt 

 had been thrown, has been confirmed by the researches 

 of the best modern observers. M. Struve measured 

 Encke s comet as it approached the sun towards the 

 end of the year 1828. He found that between October 

 28 and December 24 the comet collapsed to about the 

 sixteen-thousandth part of its original volume. Sir 

 John Herschel found in like manner that Halley s 



